The present invention relates to a method for welding hard metals directly to wear members in order to enhance their resistance to wear, the wear members being in general made of iron base metals an being for instance the inner surfaces of the outer shells of centrifugal separators, the screw blades in the inner shells of centrifugal separators, the wear members of construction machines such as wear parts of concrete pump shutoff valves, side guides of rolling mill stands and so on.
It has been long considered that it is impracticable to weld hard metals to iron base metals such as cast iron, ordinary steels and stainless steels, mainly because of the resultant thermal cracking of the hard metals, the resultant formation of structurally defective layers in the weld and the extreme decrease in strength due to the structurally defective layers. Therefore, the methods for mechanically joining hard metals to the wear members with pins, for silver brazing hard metals to the wear members or for indirectly welding hard metals to the wear members through the backing plates have been mainly employed.
For instance, in order to join hard metal segments to the screw blades in the inner shells of centrifugal separators, there have been proposed (1) the method disclosed in U.S. patent application Ser. No. 533,198 filed Dec. 16, 1974 and now U.S. Pat. No. 3,977,515 wherein a segment is inserted into a dovetail groove of a screw blade and is joined at one end to the blade with a pin and (2) the method disclosed in U.S. Pat. No. 3,764,062 wherein a hard metal segment is securely joined, by silver brazing, to a backing plate and then the backing plate is welded to the screw blade. The former is unsatisfactory in practice with respect to workability and setting strength. The latter has been widely appreciated to be useful because of easiness in operation. However, a further improvement has been desired wherein a hard metal is directly welded to wear members without the resultant thermal cracking, by selected combination of hard metal and filler metal.
An object of the present invention is therefore to provide a method for directly welding hard metals to iron base metals, the method having been so far considered impossible.
Another object of the present invention is to provide a method for easily and positively welding hard metals to wear members of for example centrifugal separators, construction machines and rolling mill stands in order to ensure long service life of machinery.
A further object of the present invention is to provide a method for welding hard metals to wear members which facilitates the replacement operation of hard metal members when such members are worn out or suddenly broken.
The present invention will become more apparent from the following description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.